1. Field of the Invention
The present invention relates to a commutating device for a small-sized motor mainly used for driving audio and video equipment, and a manufacturing method therefor. More particularly, the present invention relates to a low-cost commutating device for a small-sized motor in which the state of contact with a commutator is stabilized and wear of a brush sliding portion and the commutator is reduced, and a manufacturing method therefor.
2. Description of the Related Art
FIG. 7 is a view typically showing an ordinary small-sized motor. FIG. 7(A) is a longitudinal sectional view of the whole, and FIG. 7(B) is a view of an end cap viewed from the motor inside. A magnet 9 is provided in a metallic casing 5. A rotor is formed by assembling a rotor magnetic pole, which is formed by a laminated core 2 on a shaft 1 and a winding 3, and a commutator 4 as a unit.
After the rotor assembled as a unit on the shaft 1 is inserted through an opening in the metallic casing 5, an end cap 6 is mounted so as to close the opening in the metallic case 5. A pair of brushes 8, 8, which are in contact with the commutator 4, are attached by a brush holder 7 made of a synthetic resin, which is installed so as to fit in a hole provided in the end cap 6. The opposite ends of the brushes 8, 8 in the lengthwise direction to the commutator contact side are respectively connected to terminals 10 taken out to the outside of the end cap 6. The brushes 8, 8 typically shown in the figure are so-called metallic brushes, and each of them has a shape of plate-shaped cantilever spring. Also, the brushes 8, 8 can be configured so that fork-shaped slits are provided in the tip end portion.
An extension of the shaft 1 is projected from the bottom of the metallic casing 5 in a state in which the end cap 6 is mounted on the metallic casing 5, by which the rotor is rotatably supported by bearings provided at the bottom of the metallic casing and on the end cap 6. At this time, the brushes, 8 are arranged so as to be in contact with the commutator 4. The electric current supplied from an external power source via the brush 8 and the commutator 4 flows in the winding 3 wound around the rotor magnetic pole, by which the motor can be rotated.
Conventionally, the brush for such a small-sized motor has been formed of a material in which a brush base material consisting generally of a Cu-based alloy is clad with an AgPd alloy. The Cu-based alloy is, for example, a CuNiZn alloy. As the brush base material, an Fe-based alloy (for example, stainless steel) can also be used besides the Cu-based alloy. FIG. 8 is a view for illustrating a general manufacturing method for a brush of the related art. As shown in FIG. 8(A), a slender tape material of AgPd (silver palladium) is prepared, and on the other hand, a groove coinciding with this tape material shape is formed in the surface of the brush base material consisting of a plate-shaped Cu-based alloy. Next, after this AgPd tape material is fitted in the groove in the Cu-based alloy surface, the brush base material is finished to a clad material on which a diffusion layer is formed by rolling or the like process. Next, many brush shapes are simultaneously stamped out by pressing from a member (AgPd clad material) formed as shown in FIG. 8(B), by which brushes are completed.
By using an AgPd alloy, which is considerably more expensive than a Cu-based alloy, limitedly in a sliding portion with respect to the commutator, the material cost of brush can be reduced. However, since the fabrication cost increases as the thickness of AgPd decreases, there is a limit in reducing the cost by decreasing the thickness of AgPd. There is also a limit in decreasing the width of AgPd (length direction of slender brush).
The AgPd clad material has a problem in that since the production yield is about 70%, the production efficiency is low, and the lead time (time from ordering to completion) is long. Also, the position of AgPd in the AgPd clad material is mainly determined by the fabrication of groove, so that the dimensional accuracy is not very high.
Usually, since comparatively thick AgPd with a thickness of 5 to 10 μm is used, there is a limit in further reducing the manufacturing cost of material cost plus fabrication cost although the AgPd clad material is used for the brush of motor.
Japanese Patent Publication No. 2-59236 has disclosed a technique for forming a brush by plating, not by a clad material. A three-layer coating is formed on the surface of a Cu-based alloy by plating. A first layer is formed by depositing any of Cr, Ni, Ni alloy, and Re to a thickness of 0.1 to 10 μm, a second layer is formed by depositing any of Rh, Pt, Pd, and Ru to a thickness of 0.1 to 10 μm, and a third layer is formed by depositing any of Au, Ag, and Au—Ag to a thickness of 0.1 to 10 μm.
Although a noble metal can be deposited thin by plating as described above, there is a limit in forming a thin plating layer considering that the motor brush is slidingly used while an electric current is caused to flow between the brush and the commutator and must keep a predetermined performance even after long-term use. Therefore, when the surface of Cu-based alloy is wholly covered with a plating layer, the material cost cannot be reduced especially when a noble metal is used for plating.
Also, a commutator element, which is used in combination with the brush device, has conventionally been formed mainly by using a cladding technique. The commutator element is formed by cladding the base material surface with a noble metal such as AgCuNi. Such a clad noble metal has a limit in decreasing the thickness thereof as described concerning the brush. Moreover, considering the brush is always in sliding contact with the surface of commutator during the motor rotation, the noble metal layer cannot simply be made thin to achieve a necessary service life.